As it is well-known, the power delivered by an internal-combustion engine is a function of the amount of air fed into the combustion chamber of this engine, an amount of air which is itself proportional to the density of this air.
Thus, if high power is required, it can be provided by compression of the air before it is admitted in the engine cylinder, more commonly referred to as supercharging.
Supercharging can be carried out by a turbosupercharger. Part of the energy lost in the exhaust gas is recovered by means of a turbine placed in the burnt gas stream. This energy is used positively by a compressor to compress the intake air, which increases air filling, thus increasing engine performances.
In order to improve this air filling even further, as it is better described in patent application FR-A-2,781,011 filed by the applicant, the residual burnt gases are discharged, during the intake phase, from the combustion chamber to be replaced by supercharged fresh air, a stage that is commonly referred to as burnt gas scavenging.
This scavenging is carried out by “overlap” of the exhaust and intake valves of a cylinder in the intake phase and more precisely by opening simultaneously, for some ten crankshaft rotation angle degrees, the exhaust and intake valves of this cylinder, in the vicinity of the top dead center of the piston.
To carry out such scavenging, it is necessary to optimize the pressure difference between the air intake pressure and the burnt gas exhaust pressure in the vicinity of the top dead center of the piston. More precisely, the pressure of the fluid at the intake has to be higher than the pressure of the burnt gases present in the combustion chamber so as to drive these exhaust gases towards the exhaust valve and to replace them by supercharged fresh air admitted by the intake valve.
However, conventional turbosuperchargers which comprise a single inlet for the exhaust gas in the turbine pose a problem that is by no means insignificant.
In fact, the exhaust gases leaving each cylinder through the exhaust means are sent to the single inlet of the turbine of the supercharger through a line connecting, directly or indirectly by means of an exhaust manifold, all the exhausts of all the cylinders to the turbine inlet.
In this configuration, as can be clearly seen in FIG. 1 that diagrammatically illustrates the exhaust pressures of each cylinder at the turbine inlet, or in the exhaust manifold, as a function of the crankshaft rotation, it can be observed that, at the start of each exhaust phase of a cylinder, the measured exhaust pressure has the shape of a peak which corresponds to a sudden exhaust pressure increase at the start of the exhaust phase for some crankshaft rotation angle degrees, then to a decrease of this pressure which thereafter stabilizes at a determined value for the rest of the exhaust phase.
As it is known to the man skilled in the art, a four-stroke and four-cylinder engine works with combustion cycles for each cylinder during which the intake phase of a cylinder corresponding to the opening of the intake valve and the exhaust phase of another cylinder, during which the exhaust valve is open, start at the same time. Therefore, when the cylinder is in the intake phase with burnt gas scavenging, the exhaust gases from the cylinder in the exhaust phase communicate with the cylinder in the intake phase by means of the exhaust valve that is open to provide burnt gas scavenging. The exhaust pressure peak generated at the start of the exhaust phase hinders or even prevents discharge of the burnt gases through the exhaust valve of the cylinder in the intake phase.
To overcome this problem, it is well-known to use a specific double-inlet or double-flow supercharger technology, commonly referred to as twin-scroll supercharger. In this type of supercharger, the exhaust gas inlet at the level of the turbine is divided in two sections, a first section connected, directly or by means of a manifold, to the exhausts of part of the cylinders and a second section connected to the rest of the exhausts of the other cylinders. Each inlet section of this supercharger is connected to cylinders for which an intake phase of a cylinder and an exhaust phase of another cylinder do not occur simultaneously. Thus, when one of the cylinders connected to an inlet of the supercharger is in the intake phase with burnt gas scavenging while another one of the cylinders connected to the other supercharger inlet is in the burnt gas exhaust phase, the pressure of the exhaust gases of the cylinder in the exhaust phase cannot interact with the burnt gas scavenging during the intake phase of the other cylinder.
This twin-scroll supercharger technology, although it gives satisfaction, is of a relatively higher cost than conventional single-inlet superchargers.
The present invention is intended to overcome the aforementioned drawbacks by using a single-inlet supercharger allowing to prevent, during the scavenging stage of the cylinder in the intake phase, interactions between the cylinder where burnt gas scavenging occurs and the cylinder in the exhaust phase, in a simple and economical way.